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I don't think what you described can happen.
Even if hash rate suddenly drop very hard blocks would still be confirmed at a very slow rate (let's say 1 per hour , as more people with GPU would start mining.
It's an extreme scenario if the perfect storm happens this will be the result.
If another crash happens, if suddenly Trump comes up saying it will ban bitcoin usage, if China prosecutes Chinese miners at the same time, yeah a 90% drop in mining will happen, but! at that point, we will have far bigger problems that a block every hour till the next adjustment.
But as I said, it's a scenario of ...what if! Chances of happening are the same as mining a solo block.
But no, GPU mining to reach the next adjustment...nope
An S19 is doing 50Th, most graphic cards on the market are able to perform only 3-4 Gh/h.
The entire hash rate is around 100Exa, so to reach 10% with GPU you would need around 2-3 billion cards
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But, have thought of viewing this issue from this angle? Every miner is in a business and the idea of doing business is to be break even and become profitable.
If the mining fee (bitcoin reward) and the transaction fees used for their payment is no longer in place, how will their businesses survive? Also altering the core philosophies of the network is a breach of trust.
Two things, in the perfect world I see, a lot of the mining would be done at a loss because it helps secure a lot of wealth, so it would make sense for let's say Coinbase to power up 10k Asics at a cost of 50k a day when it has trillions in wealth stored in the chain. Second, for a period of time, we only need the price to be double that of the last halving, of course, it can't go on like this forever but we have at least 8-12 years to think of a solution. And beyond 20 years...I don't really think I would care myself that much about earthly things!